KEY POINTS: This is the first study to demonstrate an altered circadian phase shifting response in a circadian rhythm sleep disorder. Patients with Delayed Sleep-Wake Phase Disorder (DSWPD) demonstrate greater sensitivity of the circadian system to the phase delaying effects of light. Increased circadian sensitivity to light is associated with later circadian timing within both control and DSWPD groups. DSWPD patients had a greater sustained pupil response after light exposure. Treatments for DSWPD should consider sensitivity of the circadian system to light as a potential underlying vulnerability, making patients susceptible to relapse. ABSTRACT: Patients with Delayed Sleep-Wake Phase Disorder (DSWPD) exhibit delayed sleep-wake behavior relative to desired bedtime, often leading to chronic sleep restriction and daytime dysfunction. The majority of DSWPD patients also display delayed circadian timing in the melatonin rhythm. Hypersensitivity of the circadian system to phase delaying light is a plausible physiological basis for DSWPD vulnerability. We compared the phase shifting response to a 6.5-h light exposure ( approximately 150 lux) between male patients with diagnosed DSWPD (n = 10; aged 22.4 +/- 3.3 years) and male healthy controls (n = 11; aged 22.4 +/- 2.4 years). Salivary dim light melatonin onset (DLMO) was measured under controlled conditions in dim light (<3 lux) before and after light exposure. Correcting for the circadian time of the light exposure, DSWPD patients exhibited 31.5% greater phase delay shifts than healthy controls. In both groups, a later initial phase of the melatonin rhythm was associated with greater magnitude of phase shifts, indicating that increased circadian sensitivity to light may be a factor that contributes to delayed phase, even in non-clinical groups. DSWPD patients also had reduced pupil size following the light exposure, and showed a trend towards increased melatonin suppression during light exposure. These findings indicate that, for patients with DSWPD, assessment of light sensitivity may be an important factor that can inform behavioral therapy, including minimization of exposure to phase-delaying night-time light. This article is protected by copyright. All rights reserved.